Considering how many of the people react to children showing “strength”, I thought it was strange too, but it just shows that people don’t really mind the details in this world. For sure though, his parents are onto him and know he has secrets, and they want to know, but they are silently just supporting him because he’s there son anyway. They did mention that his growth was strange and it just happened while talking to the family they are staying with. It seems to me that Arthur isn’t wise enough to hide his stuff and cover his tracks. At least he knows he needs to keep it a secret.
Every time a mana core is formed, there is an explosion, the fact he blew the house is not that weird. He just did it very soon. There is a lot of thing that is weird about him, his father has always been sparring with him and he knows that he is not "used" to his body. He just.... waits for Arthur to tell him the truth, but his parents definitively know that something is off, and they question it, just not openly
I see u miss a lot detail here, i mean, there is an explanation there thats actualy answer ur concern, but well, u miss it, thats why its feel weird :)
It's... ridiculous. A 4 y.o kid teaching ppl 6 times his age? Nope, sorry. He's just... too eager to learn, at least wait until u r 5 y.o to learn how to manisfest your mana core. I wonder htf no one cares that a 2 y.o toddler is destroying a house that easily :) But it's just my thought. This is just one of the case in which they reborn/reincarnated/transmigrated as a baby and no one questions why they get "strong" WAY too fast...